The dual epidemics of obesity and type II diabetes represent an enormous challenge to our health care system. The staggering human and monetary costs of these disorders are the direct result of having inadequate treatment options that both lower glucose levels and reduce body weight. Over the last decade, novel pharmacological strategies have become available that target the GLP-1 system. Glucagon-like-peptide- 1 (GLP-1), a regulatory peptide with a broad role in the regulation of nutrient ingestion and disposition, is produced in the intestine as well as in a small cluster of neurons in the hindbrain. Plasma GLP-1 originates from the gut, but is rapidly inactivated by the ubiquitous protease DPP-4. GLP-1's very short half-life challenges the dogma that under normal circumstances endogenous GLP-1 released by the gut acts on distant receptors in the pancreas or brain. Emerging GLP-1-based therapies use two distinct strategies: 1) Long- acting GLP-1 receptor (GLP-1R) agonists that are resistant to the actions of DPP-4; and 2) Inhibitors of DPP-4 that reduce GLP-1 inactivation, effectively prolonging the activity of endogenous GLP-1. Both classes of medication are hypothesized to stimulate GLP-1R signaling, and consequently to control hyperglycemia via a common mechanism of action. However, this cannot be the case since there are clinically important yet still unexplained differences in their spectrum of effects. Most notably, while both classes of compounds improve glucose tolerance, GLP-1R agonists additionally cause weight loss while DPP-4 inhibitors do not. Given the efficacy of GLP-1R-based therapies and the growing numbers of patients being treated with them, understanding endogenous GLP-1 and how it relates to the pharmacological action(s) of GLP-1-based therapies has immediate clinical relevance. The overarching goal of the proposed research is to identify the underlying mechanisms that mediate the effects of these novel treatments for diabetic patients, and to explain their important differences. The research will use state-of-the-art mouse genetic technologies to inactivate the only identified GLP-1 receptor selectively in pancreas, visceral sensory nerves and/or the central nervous system. Genetically modified mice will be treated with both GLP-1R agonists and DPP-4 inhibitors to determine which populations of GLP-1R are necessary for specific actions of these drugs on multiple aspects of glucose metabolism and body weight regulation. We will thus be able to identify the key receptor populations that mediate the important and varied effects of these GLP-1-based therapies. Our ability to deploy second generations of these medicines and to maximize their clinical benefit depends on identifying the key underlying mechanisms. The result of the this proposal will be to simultaneously drive new insights on the role of the endogenous GLP-1 system AND to refine and optimize current and future GLP-1-based therapies to better treat patients with type 2 diabetes.